The present invention relates to a method of friction stir welding which may be used in welding, for example, aluminum alloy material frame members; and, more particularly, the invention relates to a method of friction stir welding which may be used, for example, in a case where the heights of portions of two abutting aluminum alloy frame members to be joined by welding differ.
As disclosed in Japanese application patent laid-open publication No. Hei 9-309164 (EP 0797043A2), a friction stir welding method is a method in which, by rotating a round rod (called a rotary tool) which is inserted into a welding portion of two abutting frame members to be subjected to welding and moving the rotary tool along a welding line; the welding portion of the two frame members to be subjected to the welding is thermoplastically heated so as to be plastically fluidized and welded.
The rotary tool comprises a small diameter portion to be inserted into the joint of the welding portion and a large diameter portion which is positioned outside of the small diameter portion. The small diameter portion of the rotary tool and the large diameter portion of the rotary tool are positioned on the same axis.
The large diameter portion of the rotary tool and the small diameter portion of the rotary tool are simultaneously rotated. The welding according to the friction stir welding method is applied to an abutting portion and an overlapping portion of the two frame members to be subjected to the welding, which members are made of aluminum alloy materials, for example.
Further, to the welding portion of one frame member to be subjected to the welding, there is provided a raised portion which protrudes in a direction toward the large diameter portion of the rotary tool. This raised portion of the frame member is provided integrally with an end portion in a width direction of a hollow extruded frame member.
The above-stated raised portion provides material for burying a gap which is formed between the two hollow extruded frame members during welding. When a side of the raised portion of the frame member forms an outer face of a product, for example, an outer side of a car body of a railway vehicle, after the welding the remnants of the raised portion of the frame member are cut off.
When the above-stated raised portion is provided on the extruded frame member, the raised portion can be provided to extend along an extruding direction. However, the raised portion of the frame member can not be provided at an end portion of the extruded portion; in other words, the raised portion of the frame member can not be provided at a rectangular direction against the side of the raised portion. As a result, when the extruded frame members are arranged to be orthogonal to each other and the end portions of the extruded frame members are subjected to welding according to a friction stir welding method, a raised portion exists at an end portion of one extruded frame member, however a corresponding raised portion does not exist on the other extruded frame member at the welding portion.
Accordingly, it is considered that a good welding of the two extruded frame members can not be carried out, because one of the extruded frame members does not have a raised portion at the welding portion.
The above stated fact applies generally to a case in which the height of the surfaces in the width direction of the two extruded frame members of the welding portion differ from each other; in other words, for a case in which the height of the surface in the width direction of one frame member differs from the height of the surface in the width direction of the other frame member. The typical situation is that the height of the surface in the width direction of the one frame member is higher or lower than the height of the surface in the width direction of the other frame member.